I thought of a solution. I will use git and GitHub for the source code and any other text files, and I will create a VM in the google cloud platform and use ssh to transfer data between our computers.
We just need to be careful to not override asset data.
This is going to fail catastrophically. It might be reasonable to sacrifice advanced version control functionality, but even for assets you need complete history (not only snapshots of whenever backups run, skipping intermediate states) and atomic operations (not, for example, backups that silently omit open files).